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Pulling a Cloth From Under a Beaker

A beaker is placed on top of a cloth, on top of a stool. The cloth is pulled quickly from underneath the beaker, while the beaker remains stationary. The impulse of the net force is made very small by reducing the time over which the cloth acts on the beaker. In other words, the force of the cloth does not act on the beaker long enough to accelerate it, so it does not move.

Comments (3)

Hi… I think its more like the Frictional force between the beaker and the cloth was too small and considering the total time this small frictional force, which was trying to drag the beaker along with it, was acting on the beaker was so small, the total distance the beaker moved becomes also small (remember that the beaker is quite heavy too so the total acceleration is also small.. and If you use light empty plastic beaker this trick will not work)….

Posted 4 years by subramaniann

Hi… Can any one in do this experiment and upload..

It is to show the relative velocity. It will be cool to see this rather than explaining by words.

Try to throw a ball horizontally in opposite direction of motion from a moving car (at a velocity V)or some moving platform with same magnitude of speed. Try to film this 1) from the car/moving platform (where the velocity of thrown ball will be 2V… while for the stationary observer outside this system it will be 0 or the ball will just drop virtically at the point of release…It will be cool to actually see this..

Unfortunately i dont have a car or have the courage to stand on a skate board :-P

Posted 4 years by subramaniann

In reference to Subramaniann’s comment, the effect is INDEPENDENT of the mass of the beaker. However, there is a subtlety that has to do with the torque about the center of mass of the object being pulled. Allow me to explain in two parts.

Firstly, the underlying concept of this demo is that the impulse of the force of friction on the beaker is made SMALL because is occurs over a SMALL delta-t. Thus, the change in momentum of the beaker is SMALL (Impulse-Momentum Theorem). The change in kinetic energy of the beaker, which is a function of the change in its velocity, is similarly SMALL. One may determine the distance that the beaker travels while under the influence of this force via the Work-Energy Theorem. Consequently, the distance the beaker travels is independent of its mass. This SMALL distance is a linear function of the coefficient of friction and depends on delta-t squared. (An underlying assumption is that the beaker quickly comes to rest once it comes in contact with the table/stool).

Now, the subtlety is that the force of friction produces a torque about the cup’s center of mass. The amount the cup rotates (i.e. it’s angular acceleration) will depend on how the mass is distributed (i.e. the cup’s moment of inertia about its center of mass) and where the center of mass of the cup is. Again, the angular acceleration WILL NOT explicitly depend on the mass of the cup itself. Take the following examples:

A cup with a block of ice in the bottom. This has a center of mass that is close to where the force is being applied. The angular acceleration will be minimal.

A cup (with lid) with a block of ice on top. This has the same moment of inertia about the center of mass (assuming the cup to be cylindrical), but the distance from the center of mass to where the force is applied (i.e. the moment-arm) is much greater. Thus, it will tend to rotate more.

Ultimately, all is independent of mass but dependent on how it is distributed. If you pull fast, it shouldn’t matter.

Posted 4 years by tsg

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MIT Department of Physics Technical Services Group

MIT Department of Physics Technical Services Group

Category: Science | Updated over 1 year ago

February 22, 2011 10:38
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